Semiconductor device and its manufacture method

Abstract

Multiple kinds of transistors exhibiting desired characteristics are manufactured in fewer processes. A semiconductor device includes an isolation region reaching a first depth, first and second wells of first conductivity type, a first transistor formed in the first well and having a gate insulating film of a first thickness, and a second transistor formed in the second well and having a gate insulating film of a second thickness less than the first thickness. The first well has a first impurity concentration distribution having an extremum maximum value only at the depth equal to or greater than the first depth. The second well has a second impurity concentration distribution which is superposition of the first impurity concentration distribution, and another impurity concentration distribution which shows an extremum maximum value at a second depth less than the first depth, the superposition shows also an extremum maximum value at the second depth.

1. A manufacture method for a semiconductor device, comprising steps of:
 (a) forming an isolation region in a semiconductor substrate, said isolation region extending from a surface of said semiconductor substrate to a first depth position; 
 (b) forming first and second wells of a first conductivity type in said semiconductor substrate; 
 (c) forming a gate insulating film having a first thickness on a surface of said first well and forming a gate insulating film having a second thickness thinner than the first thickness on a surface of said second well; 
 (d) forming gate electrodes on said gate insulating films; and 
 (e) forming source/drain regions in said semiconductor substrate on both sides of said gate electrodes, 
 wherein said step (b) comprises steps of: (b1) performing ion implantation commonly to said first and second wells to form a first impurity concentration distribution having a maximum value only at a depth position equal to or deeper than said first depth position; (b2) selectively performing ion implantation to said first and/or second wells to form a second impurity concentration distribution having a maximum value at a depth position approximately equal to said first depth position; and (b3) performing ion implantation only to said second well to form a third impurity concentration distribution having a maximum value at a depth position shallower than said first depth position, and 
 wherein said first well has only the first impurity concentration distribution having a maximum value at a depth equal to or deeper than said first depth position. 
 
     
     
       2. The manufacture method for a semiconductor device according to  claim 1 , wherein said step (b2) performs ion implantation to said second well to form said second impurity concentration distribution and excludes said ion implantation to the first well by masking. 
     
     
       3. The manufacture method for a semiconductor device according to  claim 1 , wherein said step (b) further forms a third well of the first conductivity type, said step (c) forms a gate insulating film having the first thickness on a surface of said third well, said step (b1) performs ion implantation also to said third well to form said first impurity concentration distribution, and said step (b2) performs ion implantation to said second and third wells to form said second impurity concentration distribution. 
     
     
       4. A manufacture method for a semiconductor device, comprising steps of:
 (a) forming an isolation region in a semiconductor substrate, said isolation region extending from a surface of said semiconductor substrate to a first depth position; 
 (b) forming first and second wells of a first conductivity type in said semiconductor substrate; 
 (c) forming a gate insulating film having a first thickness on a surface of said first well and forming a gate insulating film having a second thickness thinner than the first thickness on a surface of said second well; 
 (d) forming gate electrodes on said gate insulating films; and 
 (e) forming source/drain regions in said semiconductor substrate on both sides of said gate electrodes, 
 wherein said step (b) comprises steps of: (b1) performing ion implantation commonly to said first and second wells to form a first impurity concentration distribution having a first peak only at a depth position equal to or deeper than said first depth position; (b2) selectively performing ion implantation to said first and/or second wells to form a second impurity concentration distribution having a second peak at a depth position approximately equal to said first depth position; and (b3) performing ion implantation only to said second well to form a third impurity concentration distribution having a third peak at a depth position shallower than said first depth position, and 
 wherein said first well has only the first impurity concentration distribution having a peak at a depth equal to or deeper than said first depth position. 
 
     
     
       5. The manufacture method for a semiconductor device according to  claim 4 , wherein said step (b2) performs ion implantation to said second well to form said second impurity concentration distribution. 
     
     
       6. The manufacture method for a semiconductor device according to  claim 4 , wherein said step (b) further forms a third well of the first conductivity type, said step (c) forms a gate insulating film having the first thickness on a surface of said third well, said step (b1) performs ion implantation also to said third well to form said first impurity concentration distribution, and said step (b2) performs ion implantation to said second and third wells to form said second impurity concentration distribution.